1. Technical Field
One or more embodiments of the present invention relate to energy conversion devices of an electrostatic induction type and, specifically, to an energy conversion device of an electrostatic induction type for electric power generation by using environmental vibration or the like.
2. Background Art
In a distributed system device or portable device equipped with a sensor or an actuator, direct power feeding by wire connection (electric cords) is difficult. Therefore, as a power supply, a coin primary battery or chargeable battery is used.
However, in a TPMS (tire-pressure monitoring system) monitoring a tire air pressure of a vehicle, health monitoring of airplanes, buildings, bridges and others, and large-scale distribution management, replacement of batteries may be difficult and troublesome, and it need to change a large quantity of batteries. Thus, using a batter as a power supply is inconvenient. Moreover, for the use purpose as a cardiac pacemaker, a capsule endoscope and others, the patient has to undergo surgery to replace batteries and therefore carry a heavy physical burden. Furthermore, even in the case of a chargeable battery, there are inconveniences similar to those in replacement of batteries, regarding a recharging operation.
Therefore, in various fields including those described above, a technology of making a power supply maintenance-free (eliminating the need of recharging or replacement of batteries) is desired.
As a possibility for maintenance-free measure, there is a small-sized energy conversion device capable of extracting power from vibrations of several Hz to several tens of Hz occurring in a moving body (such as a vehicle or an airplane), a structure (such as a building or a bridge) or the like, or occurring due to the motion of an apparatus or a human (hereinafter, such vibrations are referred to as ambient vibrations).
Energy conversion devices generating power by ambient vibrations mainly come in three types: an electromagnetic induction type, a piezoelectric type, and an electrostatic induction type. In a device of the electromagnetic induction type, because the amount of power generation is proportional to the square of the frequency of vibration, it is difficult to extract power from low frequency of vibration. Moreover, the size and the weight of the device are disadvantageously large. A device of the piezoelectric type has issues in reliability, mass productivity, such as handling at the time of assembling, and cost. For this reason, expectations have been placed on a device of the electrostatic induction type.
The energy conversion device of the electrostatic induction type is suitable for obtaining power by using ambient vibration because the amount of power generation is proportional to the frequency of vibration. However, to obtain a large amount of power generation, a large amount of electric charge has to be injected into electret electrodes. Also, if the charge amount of the electret electrodes is increased, degradation in power generation efficiency due to discharge arises. However, in a conventional energy conversion device of an electrostatic induction type, these issues have not been solved, and a sufficient electric charge is not allowed to be injected into the electrets. Furthermore, degradation in power generation efficiency due to discharge is not sufficiently prevented. In the following, these issues are specifically described by taking an energy conversion device disclosed in Patent Document 1 as an example.
(Device Disclosed in Patent Document 1)
The structure of an energy conversion device 11 (an electrostatic-induction conversion device) disclosed in Patent Document 1 is disclosed in FIG. 1. In this energy conversion device 11, a plurality of strap-shaped base electrodes 13 are provided on an upper surface of a fixed substrate 12, and an electret 14 is provided on each of the base electrodes 13. Also, on a lower surface of a counter substrate 15 facing the fixed substrate 12, counter electrodes 16 are provided with the same pitch as that of the base electrodes 13.
The electrets 14 are formed by injecting an electric charge into an area near the surface of an insulating material. In particular, as a material of the electrets 14, the material is formed of a polymer having a fluorine-containing aliphatic ring structure in a main chain, thereby increasing a surface charge density. Also, in an embodiment disclosed in Patent Document 1, on an upper surface of the electrets 14, a moisture-proof film (parylene film) formed of poly-para-xylylene or its derivative is formed, thereby preventing degradation due to moisture.
In the above-structured energy conversion device 11, a load 17 is connected between two electrodes 13 and 16 and, when the counter substrate 15 is moved in a direction indicated by an arrow in FIG. 1, an opposite electric charge is electrostatically induced to the counter electrode 16 by an electric charge injected into the electrets 14 to let a current flow through the load 17, thereby allowing usage for electric power generation.
(Decrease in Amount of Electric Charge Injected)
As a method of injecting an electric charge into the electrets 14, a method by corona discharge is general. In Patent Document 1, as depicted in FIG. 2, a needle for corona discharge 18 is used. By using a direct-current high-voltage power supply 19, a high voltage is applied between the needled 18 and the base electrode 13 for discharge for a predetermined time at a predetermined voltage, thereby injecting an electric charge into the electrets 14 for charging.
However, because the electrets 14 are formed on the upper surface of the base electrodes 13, the side walls of the base electrodes 13 are exposed from the electrets 14. For this reason, as the electret 14 on the left in FIG. 2, most of the electric charge (electrons) discharged from the needle 18 are induced to the base electrode 13 to be returned to the high-voltage power supply 19 (lines with arrows in FIG. 2 each represents a flow of electric charge at the time of discharge), and the amount of electric charge to be injected into the electret 14 is decreased correspondingly.
Also, the electret 14 and the base electrode 13 are formed by a semiconductor process (a photolithography process). However, even if the electret 14 and the base electrode 13 are designed to be formed so as to have the same area, the breadth of the electret 14 on the base electrode 13 tends to vary due to alignment deviation in a manufacturing process or process error, such as overetching of the electret 14. For this reason, as the electret 14 on the right in FIG. 2, the edge of the base electrode 13 tends to appear from under the electret 14 to be exposed. In such cases, the amount of electric charge induced to the base electrode 13 is further increased, and the amount of electric charge to be injected into the electret 14 is further decreased.
(Degradation Due to Discharge)
To obtain an amount of power generation of 0.1 mW or more by using ambient vibration (on the order of 10 Hz), a gap between the upper electrode 16 and the lower electrode 13 has to be set on the order of 50 μm (refer to Non-Patent Document 1). However, when the gap is set on the order of 50 μm, even if the surface of each electret 14 is covered with a moisture-proof film formed of a parylene film having a thickness of 0.3 μm, a discharge occurs between the upper and the lower electrodes because the loaded voltage exceeds a dielectric strength voltage between the substrate with air and the moisture-proof film having a thickness of 0.3 μm, thereby disadvantageously causing the electric charge held in the electret 14 to be drawn away (refer to Non-Patent Document 2).
To increase the dielectric strength voltage, the film thickness of the moisture-proof film is increased. However, when the film thickness of the moisture-proof film is increased, cost is increased, which is inconvenient. Moreover, because parylene has a high relative dielectric constant, when the film thickness of the moisture-proof film is increased, the possibility of decreasing the amount of power generation of the energy conversion device 11 is increased. For this reason, according to Non-Patent Document 1, the energy conversion device has to be sealed with SF6. Therefore, in conventional art, the energy conversion device has to be hermetically sealed in a ceramic package or the like, which increases the price of a package of the energy conversion device.
Furthermore, because the dielectric strength voltage is defined by a distance between the upper and the lower electrodes, it is required as an anti-discharge measure to fabricate a device not exceeding the dielectric strength voltage irrespectively of the interelectrode distance. In particular, irrespectively of the distance between the upper and the lower electrodes, a discharge is desirably prevented from occurring from an electret with a surface potential of 1000 V.